1. Field of the Invention
The invention relates to a standpipe inlet design for enhancing particle circulation and reducing gas entrainment, the design being suitable for applications in fluid catalytic cracking (FCC) units and other processes, such as fluid cokers, flexicokers, and fluidized bed combustors which circulate large quantities of particulate solids between different vessels connected with standpipes and risers.
2. Description of the Related Art
In a typical Fluid Catalytic Cracking (FCC) process consisting of a regenerator, a riser reactor and a stripper, such as that shown in U.S. Pat. No. 5,562,818 to Hedrick which is incorporated herein by reference, finely divided regenerated catalyst leaves a regenerator and contacts with a hydrocarbon feedstock in a lower portion of a reactor riser. Hydrocarbon feedstock and steam enter the riser through feed nozzles. The mixture of feed, steam and regenerated catalyst, which has a temperature of from about 200.degree. C. to about 700.degree. C., passes up through the riser reactor, converting the feed into lighter products while a coke layer deposits on catalyst surface. The hydrocarbon vapors and catalyst from the top of the riser are then passed through cyclones to separate spent catalyst from the hydrocarbon vapor product stream. The spent catalyst enters the stripper where steam is introduced to remove hydrocarbon products from the catalyst. The spent catalyst containing coke then passes through a stripper standpipe to enter the regenerator where, in the presence of air and at a temperature of from about 620.degree. C. to about 760.degree. C., combustion of the coke layer produces regenerated catalyst and flue gas. The flue gas is separated from entrained catalyst in the upper region of the regenerator by cyclones and the regenerated catalyst is returned to the regenerator fluidized bed. The regenerated catalyst is then drawn from the regenerator fluidized bed through the regenerator standpipe and, in repetition of the previously mentioned cycle, contacts the feedstock in the reaction zone.
Catalyst circulation is critical to overall performance and reliability of FCC units. The main drive for catalyst circulation comes from stable and adequate pressure build-up in the standpipe. One critical element of the standpipe design is the inlet design because it determines the inlet condition of the catalyst which, in turn, affects the entire standpipe operation.
The prior art of standpipe inlet design, for both stripper standpipe and regenerator standpipe, is a conical hopper such as that shown in the open literature in Handbook of Petroleum Refining Process, second edition by R. A. Meyers, which is incorporated herein by reference. The key concept of the inlet hopper design of the prior art is that when catalyst particles are drawn from a fluidized bed into a standpipe, bubbles are also drawn together with the catalyst. The inlet hopper provides residence time for the bubbles to coalesce and grow into large bubbles before entering the standpipe. Since large bubbles have a higher riser velocity, they have a better chance to escape back into the fluidized bed, thus reducing gas entrainment into the standpipe.
However, the design concept of the prior art standpipe inlet has several disadvantages. If the inlet hopper is too small, many bubbles drawn into the inlet hopper do not have enough time to grow but flow directly into the standpipe, leading to high gas entrainment. If, on the other hand, when the inlet hopper is large enough to allow small bubbles to grow, large bubbles could form and hang stationary inside the hopper for a period of time as the bubbles try to rise against the downward catalyst flow. These large hanging bubbles can temporarily restrict catalyst flow into the standpipe. When the bubbles finally grow large enough to escape into the fluidized bed, the release of the large bubbles creates a sudden surge of catalyst into the standpipe, leading to a sudden pressure swing in the standpipe. The sequence of growing and releasing of large bubbles leads to a very undesirable condition of unstable standpipe operation. The fundamental flaw of the prior art design is that, while the objective of the standpipe inlet design is supposed to reduce gas entrainment into the standpipe, the design in fact encourages many bubbles to be drawn in. This is inherently very inefficient. Furthermore, the prior art of the inlet hopper design is a bulky structure such that in many FCC units there is not enough room to place it. A common compromise is to use either a straight pipe or an asymmetric hopper for the standpipe inlet which further exacerbates the problems described above.
Standpipe inlet geometry not only affects catalyst circulation, the entrained gas can also have a negative impact on the performance of a stripper of a FCC unit. It is common practice that the stripper includes special trays, such as shown in the invention by Johnson et al in international patent PCT/US95/09335 which is incorporated herein by reference. The special trays in the main vessel enhance the efficiency of hydrocarbon vapor stripping by steam. The spent catalyst is then transported to the regenerator through a stripper standpipe with a hopper inlet as shown in the prior art. The hopper inlet for the stripper standpipe has been shown to be rather ineffective in reducing gas entrainment. The study of Nougier et al in the Second FCC Forum (May 15-17, 1996, The Woodlands, Texas) shows that, even after intensive stripping in the main vessel, the vapor leaving the stripper still contains 20 to 25% by mole (or about 40% by weight) of hydrocarbon products. Gas entrainment from the stripper standpipe into the regenerator has two negative impacts in addition to the impact on catalyst circulation discussed above. First, the entrained gas from the stripper to the regenerator represents a loss in hydrocarbon products which could have been recovered as products. Second, the entrained hydrocarbon has to be burned in the regenerator which consumes limited air available in the regenerator and generates additional heat that has to be removed. Thus, it is essential to reduce gas entrainment into the stripper standpipe.
One objective of the instant invention is to reduce gas entrainment into standpipes by a standpipe inlet design. This will lead to increases in overall pressure build-up in the standpipe and catalyst circulation rate as well as improving standpipe stability. The reduction in gas entrainment will also reduce hydrocarbon entrainment from the stripper to the regenerator of a FCC unit, as discussed above.